Heat exchanger-expander adapted to deliver refrigeration



Sept. 20, 1966 E, HOFFMAN 3,273,356

HEAT EXCHANGER-EXPANDER ADAPTED TO DELIVER REFRIGERATION Filed Sept. 28, 1964 INVENTOR.

Thomas E. Hoffman Attorney United States Patent 3,273,356 HEAT EXCHANGER-EXPANDER ADAPTED TO DELIVER REFRIGERATION Thomas E. Hoffman, Marblehead, Mass., assignor to Arthur D. Little, Inc, Cambridge, Mass., a corporation of Massachusetts Filed Sept. 28, 1964, Ser. No. 399,530 7 Claims. (Cl. 62-514) This invention relates to a heat exchanger and more particularly to a heat exchanger which combines with its function of heat exchange the function of expansion, thus providing a simple device which is capable of furnishing refrigeration.

In the usual refrigeration apparatus, work is normally extracted from a fluid and refrigeration is developed by compressing the fluid and then rapidly expanding it to cool it and furnish refrigeration. Pre-cooling or initial cooling of the high-pressure fluid is customarily performed by 'out-of-contact heat exchange with the low-pressure cold expanded fluid. In such devices expansion is normally carried out in an expansion engine or in an expansion valve, the Joule-Thomson valve being 'a wellknown example of the latter class of apparatus used for this purpose.

Although the Joule-Thomson valve is eflicient, and in many cases irreplaceable, there are many requirements for refrigeration apparatus wherein the incorporation of a Joule-Thomson valve has distinct drawbacks, including difiiculty of construction, the necessity for purifying the fluids to prevent contaminant solidification and plugging, and the requirement of maintaining a device which may incorporate moving parts. Thus there is a real need for a simple expansion device which is easy to construct, reliable, generally not subject to contaminant plugging, and free of moving parts making it capable of operating over a long period of time without maintenance or wear.

The heat exchanger-expander of this invention, which is in effect a refrigerator, is particularly well-suited to the cooling of small localized areas, although it is not limited to such applications. There is a real need for a refrigeration device capable of furnishing refrigeration to small areas for such uses as the cooling of mechanical or optical devices, e.g., photoelectric cells and infrared detectors. There is also a need for such a device in the recently developed techniques of cryosurgery where it is necessary to be able to anesthetize extremely small areas (for example within the brain or eye) by extremely rapid cooling. Finally I may also cite the use of such a device in the cooling of microscope stages for experimental investigations.

The prior art devices which attempt to furnish refrigeration to a small localized area are limited, the one known being a device which has finned tubing wound around a cylindrical core, the channel within the tubing carrying the high-pressure fluid and the path around the fins the returning low-pressure fluid. These prior art devices have several inherent disadvantages, including being diflicult to fabricate and diflicult to adjust the flow through the two passages. Moreover, they require an outside sheath which in elfect amounts to a greater refrigeration load, introduces losses in efliciency due to linear heat transfer along the length of the sheath and core, and adds up to a relatively large weight and volume for the quantity of refrigeration produced.

It is therefore a primary object of this invention to provide a heat exchanger-expander which is capable of producing refrigeration at a localized point. It is another object of this invention to provide an apparatus of the character described which is simple to construct, easy to control and eflicient in operation. It is yet another object to provide such apparatus which is capable of developing maximum refrigeration in a minimum volume using a minimum weight of equipment. It is yet another object of this invention to provide apparatus of the character described which is capable of extremely rapid cooldown and is flexible in the refrigeration temperatures which it develops through the proper choice of fluid and operating conditions. Other objects of the invention will in part be obvious and in part be apparent hereinafter.

{The invention accordingly comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated by the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is a side elevational view, partially in crosssection, of one embodiment of the heat exchanger-expander of this invention;

FIG. 2 is a cross-section taken along line 22 of FIG. 1;

FIG. 3 is a side view of one modification of the cold end of the apparatus; and

FIG. 4 is a cross-sectional view of another modification of the cold end of the apparatus of this invention.

In the heat exchanger-expander of the present invention high-pressure fluid is introduced into a capillary which is wound externally and is in thermal contact with a cylindrical body which is hollow and through which the cold expanded fluid returns for out-of-contact heat exchange with the high-pressure fluid passing through the capillary. The parameters of the operating conditions as well as of the size, length and flow characteristics of the capillary are so chosen and adjusted that expansion of the high-pressure fluid is effected during its passage through the capillary so that it is both cooled and expanded by the time it is discharged from the capillary and returned through the low-pressure side. It is possible to define these parameters and this will be done subsequent to a detailed description of the apparatus.

Turning now to FIG. 1, it will be seen that the apparatus consists of a hollow cylinder 11) which defines within it a low-pressure passage 11 and which has wound around it a capillary tubing 12 within which is a highpressure fluid passage 13. The capillary tubing 12 is in thermal contact with the outer wall of the cylinder 10 through actual contact of its external wall and the external wall of the cylinder 10. Thermal. contact is also extended and enhanced through the solder fillets 14. It will be noted that there remains a spacing 15 between the solder fillets 14 of each of the windings of the capillary. Thus when the cylinder 10 is constructed of a high-strength, low thermal conductivity material such as stainless steel and the solder fillets are maintained apart, the axial heat transfer along the cylinder is minimized.

The cooled end of the cylinder, in the modification of FIG. 1, is sealed with an end cap 18 which defines around this cold end a small annular chamber 19 into which the cold, expanded fluid enters from the end of the capillary and is directed into the low-pressure column 11 within the cylinder 10. However, if the heat exchanger-expander is designed to fit into apparatus which in effect supplies this sealing means, then the separate end cap 18 will not be required.

Within low-pressure column 11 are positioned foraminous discs 21 which have holes 22. These discs are maintained in spaced relationship with each other and are separated by spacings 23. These foraminous discs 21 are permanently aflixed and thermally bonded to the interior wall of the cylinder 10 by means of solder 24. The f-oraminous discs 21 extend throughout the length of the column to approximately that point at which the capillary 12 breaks contact with the external walls of cylinder 10. This is illustrated by the dotted lines in FIG. 1.

For purposes of illustration, a refrigeration load is illustrated to be a small wafer attached or in thermal contact with the bottom of the sealed end cap 18. Itwill of course be appreciated that this refrigeration load can take many forms and that this is only illustrative.

Although the manner in which the apparatus of this invention may be mounted or made an integral part of other apparatus is not part of this invention, a typical way in which this may be done is illustrated in FIG. 1. In this embodiment the cylinder 10 and the capillary tubing 12 are mounted in a block and the high-pressure fluid is introduced through a high-pressure conduit 32 which communicates within the block with the opening of capillary 12 which in turn is sealed in the block with solder 34. In like manner the cylinder 10 is connected to the low-pressure fluid outlet conduit 35 within the block 30. Finally the block may be mounted in a plate 36.

FIG. 3 illustrates a modification of the end of the apparatus of this invention which eliminates the use of the seal end cap 18 which is required in the apparatus of FIG. 1 to provide the necessary passage for conducting the lowpressure fluid from the open end of the capillary 12 into the low-pressure channel 11. In the modification of FIG. 3 the cold end of the cylinder 10 is closed and the end of the capillary 12 is sealed in so that the cold expanded fluid is transferred directly from the capillary to within the channel 11. This modification makes a particularly good probe-type refrigerator.

The modification of FIG. 4 illustrates the incorporation of an outer sheath 42 around the cylinder 10. This sheath may extend the entire length of the cylinder if protection from mechanical shock and the like is required. Such a sheath has some inherent drawbacks, the most important of which is that it increases the refrigeration load on the apparatus by the introduction of an additional axial heat leak path. However, if such a sheath is required it may be used and if desired, as shown in FIG. 4, the hollow cylinder 10 may be inserted into the sheath to contact the bottom thereof. In this case suitable apertures 43 are provided in the cold end of the cylinder wall to permit the cold expanded fluid discharged from the capillary 12 into annular chamber 19 to enter low-pressure channel 11. FIG. 4 also illustrates the incorporation of a heat station 44 at the cold end of the apparatus. Such a heat station is typically a mass of copper employed as a heat sink and stabilizer. It may also be desirable to fill in all or part of the gaps between the bottom windings of the capillary 12 with an insulating-type material to provide some protection against shock. Thus in FIG. 4 several of these gaps are filled with a suitable material 20, e.g., silicon rubber. A nylon thread may also be used in which case the gap is not completely filled. This filler may also extend just beyond the capillary to prevent contact between the capillary and the sheath.

As noted above, in order to insure the eflicient performance of this appaartus as a heat exchanger and as an expander, and in order to be able to deliver refrigeration efliciently, it is necessary that certain relationships be es tablished among the various parameters of the apparatus. The parameters to be considered are the hydraulic radius and length of the capillary; the pressure of the fluid introduced into the capillary, and the pressure of the cold expanded fluid discharged by the capillary; and the friction characteristics of the internal capillary walls, the mean temperature and the fluid viscosity. The refrigeration delivered is proportional to the mass flow of the fluid and this in turn is dependent upon these parameters. By controlling the mass flow the amount of refrigeration can be controlled; and this control of mass flow is most readily effected through the control of the hydraulic radius which may be defined as 4 times the cross-sectional flow area divided by the wetted perimeter. For this reason this parameter (hydraulic radius) is used in place of the actual radius or diameter of the capillary, a characteristic normally associated with a truly circular cross-section.

In the design and construction of a refrigeration apparatus in accordance with this invention it will be most convenient to fix mass flow (which is determined by the refrigeration desired) pressures, mean temperature and the hydraulic radius of the capillary tubing. The friction factor is of course an inherent property of the capillary tubing. Given these parameters, it is then possible to determine the length, L, of the capillary tubing which will give the desired performance of the device. This may be expressed as where L=the length of the capillary in inches P =pressure of the high-pressure fluid at the point of introduction into the capillary in pounds force per square inch T =temperature of the high-pressure fluid at the point of introduction into the capillary in Rankine P zpressure of fluid after expansion, i.e., the pressure of the cold expanded fluid entering the low-pressure return path in pounds force per square inch T =temperature of fluid after expansion, i.e., the temperature of the cold expanded fluid entering the low-pressure return path in Rankine T: a predetermined mean temperature between T and T in Rankine r =hydraulic radius in inches F=friction factor (non dimensional) R=universal gas constant in inch pounds force per pounds mass per degree Rankine g=gravity constant in inch pounds mass per second squared per pounds force w mass flow in pounds mass per second The units given are typical of those used, being in this case in the English system. Other units can, of course, be substituted in other systems.

It will be seen from the above relationship that for a given length of capillary the flow rate varies as the 5/2 power of the hydraulic radius so that small changes in the hydraulic radius can be used to accurately adjust this flow rate. This is most conveniently accomplished by applying pressure evenly around the capillary tubing wound about the cylinder in order to flatten the capillary tubing, thus changing its hydraulic radius by very small but accurately controlled amounts. It can be seen that minor changes in hydraulic radius can be used to bring about changes in the mass flow rate and hence once the apparatus is assembled it is easy to adjust.

It is essential in the construction of the heat exchangerexpander of this invention to choose construction materials which will maximize transverse heat conductivity and minimize axial or linear heat conductivity. The cylinder 10 is preferably constructed of a material which exhibits both good strength and low heat conductivity in order that it give to the apparatus suflicient strength and at the same time minimize heat transfer along its length. Stainless steel may be cited as an example.

The capillary tubing should be formed of a material which exhibits good thermal conductivity. This may be a single metal such as nickel or a metal which in itself is not a good heat conductor but which is plated externally with a material which is a good heat conductor. Thus the capillary tubing can also be conveniently formed of stainless steel which is plated with gold, silver or copper.

The foraminous discs are conveniently formed of screening or thin perforated plates made of a material which has a high heat conductivity over the temperature range which the apparatus operates. They may be die cut from screening or they may be photo etched to form discs which have a narrow solid annular ring defining the periphery. The solder is preferably one which exhibits good thermal conductivity, e.g., a soft solder such as a tin-lead solder.

In constructing the heat exchanger-expander it is preferable to coat the internal walls of the cylinder with soft solder. The foraminous discs are then inserted into the cylinder and their position located by the extent to which they are pushed down into the cylinder. Their size is such that they make a tight fit with the walls and when the cylinder containing the discs is subsequently heated to cause the solder to flow, the discs will be permanently adhered to the interior of the column in the manner illustrated in FIG. 1. The capillary tubing is wound about the outside of the cylinder 10, the solder being applied in any suitable manner. If the solder fillets bridge the gap between them, the solder forming the bridge may be scraped away.

An examination of FIG. 1 will illustrate that substantially all of the heat transfer takes place along the path which includes the foraminous disc, the solder holding it to the internal wall, the thickness of the wall, the solder fillets associated with the capillary and finally the capillary wall itself. Thus this heat transfer is primarily transverse rather than axial.

The heat exchanger-expander of this invention may of course be constructed in a variety of sizes and configurations. It is, however, particularly suitable for very small apparatus to deliver refrigeration over a pinpoint area. As an example of the performance of such an apparatus, the following may be cited. Capillary tubing formed of nickel, drawn to have an inside diameter of 0.005 inch in a length of 14 inches, was wound around and soldered to a cylinder of stainless steel having an outside diameter of 0.094 inch. Perforated 0.003 inch thick copper screen having 0.008 inch diameter holes on 0.009 inch centers cut as discs were used to form the foraminous discs which were soldered into the cylinder, some fifty discs being used over a length of approximately one inch. The assembly was then inserted into a closed end tube which was part of another apparatus. This then was equivalent to sheath 42 of FIG. 4.

Freon at room temperature and 1500 psi. was then introduced into the capillary and in its passage through the capillary expanded to approximately 2 psi. Refrigoration at 80 C. was delivered by this apparatus and it required only one second to achieve cool-down to this temperature.

Any pressurized gas which exhibits a positive Joule- Thomson effect may be employed. By staging two or more of these units and using each preceding unit to initially cool the pressurized fluid which is to be expanded in any one stage, it is possible to use fluids such as helium which, at room temperature, have a negative Joule- Thomson effect. This initial cooling should be sutficient to reduce the temperature of the high-pressure fluid entering the capillary to somewhat below that temperature at which the fluid exhibits a positive Joule-Thomson effect. In this manner it is possible to deliver refrigeration to below 4 K.

It will be seen from the above description that there is provided a unique heat exchanger-expander capable of delivering refrigeration in an eflicient manner. The apparatus exhibits extremely fast cool-down, is subject to accurate, simple control and is flexible with respect to its performance. Because it does not require an outer protective sheath, and because the low-pressure path is within the apparatus, it does not require an outer cylinder to define a heat transfer path around the capillary. By eliminating this outer cylinder it is possible to speed up cool-down because it reduces the thermal mass which must be cooled. It also gives more net refrigeration because it reduces heat leak which would take place in the outer casing. Finally it also reduces overall total weight and space requirement, making it possible to fit the apparatus of this invention into many different devices Where space is a premium.

It will thus be seen that the objects set forth above among those made apparent from the preceding description are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all the generic and specific features of the invention herein described and all statements of the scope of the invention which as a matter of language might be said to follow therebetween.

I claim:

1. An apparatus suitable for providing refrigeration to a localized area, comprising in combination (a) a hollow cylinder defining a fluid channel therein;

(b) spaced foraminous discs within said channel affixed to the internal wall thereof and positioned normal to the direction of fluid flow;

(c) tubing helically wound around the external surface of said cylinder along substantially its entire length and permanently afiixed thereto, the external surface of said tubing being in direct heat transfer contact with said external surface of said cylinder;

(d) means associated with one end of said tubing for introducing a fluid at pressure P and temperature T thereto;

(e) means associated with the other end of said tubing for conducting said fluid leaving said tubing into said channel to be returned in out-of-contact heat exchange with said fluid flowing through said tubing wherein the external .path for said heat exchange com-prises said foraminous discs, the wall of said hollow cylinder and the wall of said tubing; the hydraulic nadius r and length L of said tubing being of such dimensions that said high-pressure fluid in its passage therethrough is expanded and cooled to a predetermined pressure P and temperature T whereby refrigeration is available; said hydnaulic radius and said length lbeing determinable from the relationship L fRT 6410 wherein f is the friction factor characteristic of said tubing, R is the universal gas content, T is a mean temperature between T and T g is the gravity constant, and w is the mass flow of said fluid.

2. An apparatus in accordance with claim 1 wherein said tubing is aflixed to said external surface of cylinder by solder fillets on each side of said tubing, the solder fillets on adjacent windings of said tubing being separated to prevent thermal contact therebetween.

3. An apparatus in accordance with claim 1 wherein said tubing at said other end is open and said means for conducting said fluid into said channel comprises a fluidtight sealing cap over said other end.

4. An apparatus in accordance with claim 1 wherein said other end of said tubing communicates directly with said channel thereby providing said means for conducting said fluid into said channel.

5. An apparatus in accordance with claim 1 wherein at least a portion of the spacing between the windings of said tubing is filled with a thermally insulating material thereby to provide protection against shock.

6. An apparatus in accordance with claim 1 further characterized !by having a protective sheath surrounding at least a portion of said tubing.

7. An apparatus in accordance With claim 6 wherein 3,018,643 1/1962 Evers 62-514 X 3,055,191 9/1962 Dennis 62-514 X 9/1965 Geist et a1 62511 X said sheath serves as said means for conducting said fluid into said channel. 3,205,679

ROBERT A. OLEARY, Primary Examiner.

References Cited by the Examiner 5 UNITED STATES PATENTS FREDERICK L. MATTESON, JR., Examiner.

2,791,104 5/1957 Duz 13 X M. A. ANTONAKAS, Assistant Examiner.

7/1959 Streeter 62514 X 

1. AN APPARATUS SUITABLE FOR PROVIDING REFRIGERATION TO A LOCALIZED AREA, COMPRISING IN COMBINATION (A) A HOLLOW CYLINDER DEFINING A FLUID CHANNEL THEREIN; (B) SPACED FORAMINOUSD DISCS WITHIN SAID CHANNEL AFFIXED TO THE INTERNAL WALL THEREOF AND POSITIONED NORMAL TO THE DIRECTION OF FLUID FLOW; (C) TUBING HELICALLY WOUND AROUND THE EXTERNAL SURFACE OF SAID CYLINDER ALONG SUBSTANTIALLY ITS ENTIRE LENGTH AND PERMANENTLY AFFIXED THERETO, THE EXTERNAL SURFACE OF SAID TUBING BEING IN DIRECT HEAT TRANSFER CONTACT WITH SAID EXTERNAL SURFACE OF SAID CYLINDER; (D) MEANS ASSOCIATED WITH ONE OF END OF SAID TUBING FOR INTRODUCING A FLUID AT PRESSURE P1 AND TEMPERATURE T1 THERETO; (E) MEANS ASSOCIATED WITH THE OTHER END OF SAID TUBING FOR CONDUCTING SAID FLUID LEAVING SAID TUBING INTO SAID CHANNEL TO THE RETURNED IN OUT-OF-CONTACT HEAT EXCHANGE WITH SAID FLUID FLOWING THROUGH SAID TUBING WHEREIN THE EXTERNAL PATH OF SAID HEAT EX- 